kitdashit,
I have the answer to your question ;D: put an infrared led at the top of your bottle, use a camera (regular web camera, strip off IR blocker and put on IR band pass filter, see this http://nuigroup.com/forums/viewthread/2953/
) This way your camera only sees the infrared led and could tell where the bottle is.
You may use the driver on codelaboratories.com for a sony ps3 eye camera and follow one of many online tutorials to mod the camera to only see the infrared led, which is the position of your bottle!) Check out discussion forum on processing.org for how to capture image and process it to find the led position.